builder_config: add GCE_TARBALL to ArtifactTypes

Also fixup crrev/c/2507285 where type 28 was not reserved.

BUG=b:168762436
TEST=generate.sh

Change-Id: I0c1750e1014cb652792269050a9521ea39a8b829
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2536511
Commit-Queue: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Auto-Submit: Shao-Chuan Lee <shaochuan@chromium.org>
4 files changed